It is known that certain materials such as silicon, silicon-nitride and indium phosphide have a significant thermo-optic coefficient. This means that the index of refraction of the material changes with a change in temperature. If a filter is constructed using such material, the filter spectrum will typically shift in wavelength as the temperature changes. This thermo-optic coefficient is about 0.1 nm/° C. for silicon, about 0.02 nm/° C. for silicon-nitride, and about 0.1 nm/° C. for indium phosphide.